Many systems exist for delivery of notification messages. They are useful for both business applications (e.g., delivering updates on a software bug tracking system, delivering stock quotes, delivering appointment reminders) and consumer applications (e.g., delivering news updates, delivering sports results, delivering event reminders). In the past, systems delivered reminders either via a push system (e.g., simple mail transfer protocol (SMTP) email transfer, short message service (SMS)) or a user retrieval based system (e.g., feeds using resource description framework site summary (RSS) or Atom format for web feeds). Delivering messages in either of these ways is relatively straightforward, a content provider for a push system needs to maintain a user list and deliver the message to all users; a content provider for a user retrieval system need only publish the messages in a predetermined location using a predetermined format. Modern users demand messages delivered to their mobile smart phones. Different smart phone manufacturers each provide a notification delivery service for their branded phones, each notification delivery service operating using different protocols and different message types, and often provided as a service by the phone manufacturers or phone operating system manufacturers themselves.
Some systems for delivery of notification messages, rather than working with relatively trivial notifications (e.g., sports results, activity reminders, etc.), deal primarily with crucial notifications (e.g., corporate meeting reminders, network system administrator emergency messages, etc.) that must be guaranteed to arrive.